In recent years, personal computers and various electronic devices have been able to handle a large volume of data such as moving images and sound, and therefore mass information storage devices have been needed. Accordingly, the demand for an information recording medium with a high recording density is increasing year by year. To meet the demand, a hard disk employing a perpendicular magnetic recording system has been put into mass production. In the perpendicular magnetic recording system, a substrate for an information recording medium (also referred to as a “hard disk substrate” in the following) is required to have high levels of heat resistance and surface smoothness, as compared to the existing substrate. Further, it is more important than ever for the substrate to achieve a low specific gravity in order to reduce the load on a spindle motor, a high mechanical strength in order to prevent the crush of a disk, and high fracture toughness in order to withstand the impact of a head that may accidentally fall down.
The material used for the substrate for an information recording medium is, e.g., an aluminum alloy or glass. Recently, glass has been widely used for possible dynamic applications, since glass is superior to the aluminum alloy in both Vickers hardness and surface smoothness.
Moreover, in recent years, various recording systems have been developed to address the demand for a higher recording density (see Patent Documents 1 and 2).
Patent Documents 1 and 2 disclose a hard disk substrate for a heat-assisted magnetic recording system using a laser or the like. In this case, the hard disk substrate is an amorphous glass substrate.
Further, various cleaning compositions have also been developed to address the demand for improved surface smoothness (see Patent Documents 3 to 6).
Patent Document 3 discloses a method for cleaning a crystallized glass substrate. The method uses a cleaning agent that contains an inorganic alkali and an amine including 1 to 10 nitrogen atoms to clean polishing particles such as silica particles. Thus, the method can reduce the deterioration of the surface roughness of the crystallized glass substrate during the cleaning, and further can improve the detergency. Patent Document 4 discloses a cleaning agent for electronic materials such as a hard disk substrate. This cleaning agent contains an anionic surface active agent and an organic solvent, and is effective in removing particles without sacrificing the smoothness of the surface of a substrate. The cleaning agent also has low foaming properties and excellent stability over time. Patent Document 5 discloses a cleaning agent for electronic materials such as a magnetic disk substrate. This cleaning agent contains a sulfate ester salt, an alkali, a chelating agent, and water, and is effective in removing particles. Patent Document 6 discloses a cleaning agent for electronic materials such as a magnetic disk substrate. This cleaning agent contains a nonionic surface active agent, a high molecular type anionic surface active agent having an aromatic ring and a sulfonic group, and water. The cleaning agent has very high detergency for abrasive grains even at a high temperature.